The present invention relates generally to the field of mailing machines for printing a postage indicia on an envelope to evidence the payment of postage, and more particularly to a mailing machine having a thermal printer in which the postage indicia is generated on the envelope by the process known as thermal transfer printing.
Mailing machines have long been well known, and are found both in the art and in commercial use in a wide variety of configurations and incorporating several forms of printing devices. At the present time, the great majority of mailing machines in use include printing devices which utilize the well known ink offset printing technology, in which ink is applied to a suitable printing die which has been engraved with the text and associated graphic material constituting the postage indicia, and from which the ink is transferred from the die to the surface of an envelope that is pressed against the die. This technique involves several disadvantages, one being that it requires the rather laborious and expensive procedure of engraving various forms of dies with intricate graphic and text material by well known engraving processes. In the majority of configurations of postage meters, the printing die is mounted on a rotary drum which forms part of the feeding mechanism for the envelopes. The dies, therefore, must conform to the curvature of the drum, which renders the engraving process all the more difficult and expensive.
Another disadvantage of this offset printing technique is that it requires a rather complicated inking apparatus to apply an even coating of ink to the printing die after each printing operation, which further adds to the complexity and cost of the mailing machine, and also involves constant maintenance. Thus, this printing technique is most suitable for installations such as large mailers in which the mailing machine will be used for consistently lengthy periods of time to make perhaps thousands, if not tens of thousands, of imprints. The difficulty and cost of making printing dies for use in the ink offset printing technique renders this technique unsuitable and impractical for postage meters intended for use over a limited period of time by relatively small users.
A further disadvantage of the offset printing technique arises in connection with the use of advertising slogans, which are printed adjacent to the postage indicia by another printing die mounted on the rotary drum. These dies tend to be costly since they are made up in small quantities, and may even be made as a single die on custom order for particular customers who may wish to change their advertising slogan more frequently than is economically possible with engraved dies.
It is thus apparent that there is a need for a relatively simple and inexpensive mailing machine having a postage meter printing device which is based on a printing technology that is readily available, and which can be incorporated in a postage meter printing device that is relatively simple in construction, inexpensive to manufacture, reliable in operation easy to operate and maintain, and which prints a sufficiently high quality postage indicia that will meet the stringent standards for postage indicia set by the United States Postal Service.
To meet this need, a well known printing technique has been modified for use in mailing machines which entirely eliminates the need for relatively expensive engraved printing dies and the complex and expensive inking apparatus that is required for use with such dies. This technique is thermal printing, which has been utilized for some time in various forms of printing applications, such as calculators, cash registers, facsimile machines, etc., in which a specially designed thermally responsive paper is selectively heated in a particular configuration to produce the desired image. In a mailing machine, however, where it is desired to print the postage indicia on plain paper, the image must still be deposited in the form of ink which is impregnated into the paper.
In order to get an ink image onto plain paper using a thermal process, it is necessary to supply the ink, which is a dye in a wax, polymer or wax and polymer base, in the form of a ribbon consisting of a very thin plastic film, similar to that used for wrapping food, on which is deposited a layer of wax which contains a suitable dye to impart a desired color to the wax. The ribbon is passed over a printing surface which contains an array of extremely small high temperature resistor type heaters, each individually actuated by a software-controlled microprocessor. As an envelope moves across the printing surface with the surface of the envelope in contact with the ink ribbon, individual heaters are actuated in a predetermined sequence under the control of the microprocessor to deposit small dots of ink on the surface of the envelope, thereby forming the desired image of the postage indicia.
A mailing machine having a thermal printing device as just briefly described has several distinct advantages over the more conventional die and ink printing devices described above. One major advantage is that the image printed by a thermal printer postage meter is determined by a software controlled microprocessor, which energizes the array of heaters in a sequence previously programmed into the software. The image to be printed can thus be changed at will merely by changing the software. This capability has the distinct advantage that the same postage meter can be used in any country in the world merely by providing software that has been programmed to print an image corresponding to the image of the standard postage indicia for a particular country. This is both far more convenient and economical to the manufacturer than having to provide differently engraved plates for each country in which the mailing machine might be marketed.
In like manner, mailing machine customers who wish to print a personalized slogan, such as advertising, commemorating a holiday, promoting a cause, or other personal message, together with the postage indicia, can do so with a thermal printer postage meter merely by obtaining software programmed for the particular slogan. Again, as with the postage indicia dies, it is more convenient and economical for customers, especially small volume users, to procure software when they desire to change their slogans than it is to procure different engraved dies.
Some other important advantages of the thermal printing device are that they are far less complex in structure, and hence much more economical to manufacture, easier to install and maintain, and require less service than a conventional postage meter. All of these factors contribute significantly to making a thermal printer postage meter more economically appealing to small and medium size mailers than the conventional postage meter.
A major problem, however, that occurs with this type of thermal printer is the possibility that wax ink will be transferred from the ink ribbon to a pressure roller that is mounted on the side of the envelope opposite the surface thereof on which the postage indicia is printed. The ink ribbon is normally packaged in a cartridge and is mounted on a supply spool, from which it is passed across the printing surface containing the heaters and is taken up on a driven take up spool. The take up spool is driven only for the purpose of maintaining the ribbon under tension between the spools, and not for the purpose of pulling the ribbon across the printing surface when it is in contact with an envelope during a printing operation. This would probably break the ribbon. Therefore, the ribbon is moved across the printing surface merely by the friction imposed between the envelope and the ribbon by a back up pressure roller which is maintained in contact with the opposite side of the envelope and engages it with sufficient friction to move the envelope and the ink ribbon across the printing surface.
In this arrangement, it should be apparent that if any wax based ink is deposited on the back up pressure roller, it will act like a lubricant on the surface of the roller to substantially reduce the coefficient of friction between the rubber surface of the roller and the surface of the envelope, thereby resulting in slippage of the envelope and the ribbon and consequently the printing of a postage indicia of illegible and unacceptable quality. This problem is exacerbated by the fact that it is virtually impossible to sufficiently clean wax from the porous surface of a rubber feed roller to restore the roller to its original coefficient of friction, with the result that if wax based ink is deposited on the roller from the ink ribbon, the roller must be replaced, which usually requires a service call and results in down time of the mailing machine to the customer. These problems do not exist in the conventional ink offset type printing devices because the rotary drum with the curved printing dies, acting in cooperation with a back up roller, contributes to the feeding of the envelope, with the result that the envelope is contacted on one side by the pressure roller and on the other side by the printing die, thereby assuring proper feeding of the envelope even if ink somehow is deposited on the back up pressure roller. The worst consequence of this is that the pressure roller will deposit a smudge of ink on the back of the envelope.
It is therefore apparent that there is a need for a thermal printer having certain features of control over the operation thereof that it becomes impossible for the printer to operate in the event that any part of the backup pressure roller is exposed to the ink ribbon, either during passage of an envelope through the printer, or before an envelope has entered into or exited from the printer. Thus, the present invention is directed to providing control components that will effectively control the operation of the printer to prevent operation thereof when certain circumstances exist under which it would be inappropriate for the printer to operate. The present invention also contemplates a method of detecting the precise location and orientation of an envelope within a printer as it moves through a succession of processing locations to enable the control components to effectively control the operation of the printer.